The present invention relates to an ink jet printing or recording apparatus comprising means for causing an ink ejection nozzle to eject pressurized ink having a determined frequency of pressure oscillation, charging ink droplets by means of a charging electrode at a position where the ejected ink separates into droplets, and guiding the charged ink droplets onto a recording sheet or a gutter for printing. More particularly, the present invention is concerned with a method of setting the timing for charging the ink droplets.
Various types of ink jet recording apparatus have been proposed (e.g. IBM Technical Disclosure Bulletin, Vol. 16, No. 12 (May 1974) pages 3877 and 3878 Japanese Patent Publication No. 47-43450 and Japanese Unexamined Patent Publication No. 50-46450). In a recording apparatus of the type described, if the timing for forming an ink droplet and that for applying a charging voltage (pulse) to the charging electrode deviate from each other, the ink droplet fails to receive a desired amount of charge and becomes displaced on the sheet from a desired position causing distortion of the reproduced image. This problem has heretofore been coped with by searching for a proper charging phase and thereby properly determining the timing for applying a charging voltage to the charging electrode. Such expedients are disclosed in Japanese Patent Publication No. 47-43450 and Unexamined Publication No. 50-60131 for example.
According to the conventional techniques for setting the charging timing, the charge on an ink droplet is detected by means of a charge detecting electrode by progressively shifting the phase of a search pulse which has a shorter period than a charging pulse. After the proper search pulse phase has been determined, the phase of a recording charge pulse (having a period longer than that of the search pulse) is set so that its substantially intermediate portion coincides with the search pulse. Then the recording operation is initiated. Such a procedure, where a plurality of ink ejection nozzles are arranged to perform multi-spot simultaneous recording, requires one charge detecting electrode and one charging electrode for each nozzle. The charge detecting electrodes and charging electrodes cannot be easily installed and must be shielded to avoid mutual interference. Moreover, large numbers of charge detection circuits and charge control circuits must be installed in one-to-one relation with the charging electrodes and charge detecting electrodes. Another drawback is that, since the individual heads have different phases of separation of ink into droplets, it is difficult to synchronize them with print data.